Def nnsub(m;n) == if m<n then 0 else m-n fi 

is mentioned by

Thm* m,n:. mn  nnsub(n;m) = n-m	[nnsub_nuprl]
Def sub == m:. n:. nnsub(m;n)	[hsub]

Try larger context: HOLlib IF YOU CAN SEE THIS go to /sfa/Nuprl/Shared/Xindentation_hack_doc.html